NIPT Timing Across Pregnancy
NIPT can be performed starting at 10 weeks gestation and technically throughout pregnancy, but it is not recommended or validated for use in the second and third trimesters as a primary screening tool. 1
Optimal Timing Window
NIPT should be performed after 10 weeks gestation, when approximately 10-15% of total cell-free DNA in maternal plasma is of placental origin, providing sufficient fetal fraction for accurate analysis. 1, 2
The test can be conducted as early as 9-10 weeks, though earlier testing (9 weeks) carries a higher risk of "no-call" results due to insufficient fetal fraction. 2
First trimester use (10-14 weeks) is the standard and evidence-based application of NIPT, with detection rates for trisomy 21 of 99% and false positive rates of 0.5% in women who receive results. 1
Why NIPT Is Not Used in Later Trimesters
Risk assessment has transitioned to the first trimester using NIPT and nuchal translucency measurements, making later screening less relevant from a clinical decision-making standpoint. 1
The primary value of NIPT is early reassurance or early diagnosis, allowing time for diagnostic confirmation and reproductive decision-making. 1
Earlier screening provides greater safety and lower costs if pregnancy termination is elected, and allows for safer selective reduction in twin pregnancies when performed in the first trimester. 1
Technical Considerations
If initial NIPT at 9 weeks results in a "no-call," repeat testing at a slightly later gestational age provides results in approximately 75-80% of cases. 1, 2
Maternal factors affecting test performance include obesity (associated with low fetal fraction and up to 20% test failure in high BMI women), anticoagulant use, and autoimmune disorders. 1, 2
NIPT remains a screening test, not diagnostic, and all positive results require confirmation with chorionic villus sampling (CVS) or amniocentesis regardless of when performed. 1, 2
Clinical Algorithm
For pregnancies presenting in first trimester (before 14 weeks):
- Offer NIPT at 10 weeks or later for optimal fetal fraction. 1, 2
- If performed at 9 weeks and "no-call" occurs, repeat at 10-11 weeks. 2
For pregnancies presenting in second or third trimester:
- NIPT is not the appropriate screening modality. 1
- Standard anatomic ultrasound screening at 18-22 weeks is recommended for structural anomalies. 1
- If aneuploidy screening is desired, proceed directly to diagnostic testing (amniocentesis) rather than screening. 1
Critical Caveat
NIPT cannot replace ultrasound for detection of structural anomalies and does not screen for neural tube defects, making it complementary to, not a replacement for, anatomic ultrasound evaluation regardless of timing. 1